This invention relates generally to optical focus detectors and, more particularly, to electrooptical detectors for providing an indication of the focus error with respect to a radiant energy beam impinging on a surface. Such focus detectors are useful in a variety of applications, but are particularly well suited for use in automatic focus control systems, such as might be used in recording or playback apparatus in which information is recorded on, or played back from a recording surface by means of a beam of light or other radiant energy.
By way of a more specific example, one type of disc recording system, used for recording video or other information, records information on a disc surface by means of a laser beam focused onto the recording surface by an objective lens, the intensity of the beam being modulated in some manner to produce corresponding changes in an optical condition of the surface. During playback of the recording, a laser reading beam is also employed and is focused on the recording surface as the disc is rotated. A reflected portion of the reading beam is then directed to a photodetector device, which provides an electrical signal from which the recorded information can be reproduced. In both the recording and playback modes of operation, accurate control of the focus of the beams is critical to successful operation of the apparatus.
Focus detectors of the prior art have utilized a central disc and concentric photodetector elements to measure the focus error of a beam. However, such devices do not exhibit the precision and linearity needed for some applications. Accordingly, there is still a significant need for a precise focus detector capable of measuring both the magnitude and direction of focus error. Ideally, such a detector should have a substantially linear response characteristic for some distance on both sides of the point of zero focus error, i.e., the output signal generated by the detector should be practically proportional to the focus error, which can be measured as a distance of displacement of the objective lens. The present invention satisfies this need.